In a measuring device (e.g., an oscilloscope), measurement signals are supplied via different coupling paths to a measurement amplifier. As illustrated by way of example in FIG. 1, the substantial coupling paths are the direct-current (DC) voltage coupling, which takes place via a direct signal line 1 with the non-inverting input 2 of the measurement amplifier 3; the alternating-current (AC) voltage coupling, which takes place via a signal line 5 provided with a capacitor 4 for the blocking of DC-signal components, with the non-inverting input 2 of the measurement amplifier 3; and the ground coupling, in which the non-inverting input 2 of the measurement amplifier 3 is connected to a ground 6. The selection by which the individual coupling paths are connected to the non-inverting input 2 of the measurement amplifier 3 takes place via switches 71, 72 and 73.
Further, the non-inverting input 2 of the measurement amplifier 3 can be additionally connected to the first terminal 8 of a resistor 9, of which the second terminal 10 is connected to the ground 6. With the selected impedance value of this resistor 9, the input impedance of the circuit can be adjusted in a targeted manner (e.g., to a system impedance of 50 ohms).
For the compensation of an internal offset voltage in the measurement amplifier 3, and of a DC-signal component in the measurement signal, the inverting input 11 of the measurement amplifier 3 can be connected, as in the case of the publication EP1688748A1, to the first terminal 12 of a voltage source 13 adjustable with regard to its voltage level, the second terminal 14 of which is connected to the ground 6.
Through the compensation, a displacement of the measurement signal to the zero point of the dynamic range of the measurement amplifier 3, and accordingly to the optimal operating point of the measurement amplifier 3, in which the measurement amplifier 3 comprises its maximum linearity, takes place.
This offset compensation, however, presents a disadvantage in that the voltage level of the voltage source is restricted to the maximum possible voltage level specified by the common-mode range of the measurement amplifier 3. Added to this, the linearity range of the measurement amplifier 3 is reduced by comparison with its common-mode range, so that a measurement amplifier 3 not operated at the optimal operating point (e.g., a measurement amplifier 3 that is operated with a common-mode voltage not equal to zero) causes a distortion of the measurement signal.
What is needed, therefore, is an approach for the compensation of the offset voltage present in a measurement amplifier, and of the DC-signal component present in a measurement signal, which can also compensate relatively high offset voltages in the measurement amplifier and relatively high DC-signal components in the measurement signal accurately.